Phase balance of duplex stainless steel pipes welding
It is often said that the phase balance of duplex stainless steel is "50-50", which is equivalent to the amount of austenite and ferrite. Strictly speaking, this is incorrect because the ferrite content in modern duplex stainless steel is about 40% -50%, and the rest is austenite.
It is generally believed that when the ferrite content is at least 25-30% and the rest is austenite, duplex pipe has unique advantages.
In some welding methods, especially in the method of protecting the flux for duplex 2205 pipe, by adjusting the phase balance, the austenite content in the weld can reach a higher level, thereby improving the toughness of the weld and making up for the oxygen content of the weld caused by the flux Increased loss of toughness.
The toughness of these filler metals after solution treatment is much lower than that of steel plates or steel pipes, but the toughness of the weld metal is still sufficient to meet the expected requirements.
No welding method can make the toughness of the weld metal reach the height of the wrought metal after full annealing. If the ferrite content of the weld metal is limited to the minimum required for annealing duplex stainless steel in the rolling mill, the available The welding method imposes unnecessary restrictions.
The phase equilibrium of the heat affected zone, that is, the original forged steel or stainless steel pipe plus an additional welding heat cycle, is usually slightly higher than the original material. It is almost impossible to determine the phase equilibrium of the heat affected zone by metallographic method.
If the ferrite content in this region is high, it may indicate an abnormal condition of extremely rapid cooling, which results in excessive ferrite content and reduced toughness.